docs: def ashrae_revised_clear_sky(altitudes, tb, td, use_2017_model=False):
      """Calculate solar flux for an ASHRAE Revised Clear Sky ("Tau Model").
  
      By default, this function returns clear sky values following the
      methods originally published in the ASHRAE 2009 HOF.
      Args:
          altitudes: A list of solar altitudes in degrees.
          tb: A value indicating the beam optical depth of the sky.
          td: A value indicating the diffuse optical depth of the sky.
          use_2017_model: Set to True to use coefficients associated with
              the new version of the Tau model released in the 2013 and 2017 HOF.
              Note that the correct use of the new version requires updated
              tb and td values. At present, all .stat files
              distributed by the US DoE are using the older (2009) values
              for tb and td and so this input defaults to False.
  
      Returns:
          dir_norm_rad: A list of direct normal radiation values for each
              of the connected altitudes in W/m2.
          dif_horiz_rad: A list of diffuse horizontall radiation values for each
              of the connected altitudes in W/m2.
      """
      dir_norm_rad = []
      dif_horiz_rad = []
  
      if use_2017_model is True:
          ab = 1.454 - (0.406 * tb) - (0.268 * td) - (0.021 * tb * td)
          ad = 0.507 + (0.205 * tb) - (0.080 * td) - (0.190 * tb * td)
      else:
          ab = 1.219 - (0.043 * tb) - (0.151 * td) - (0.204 * tb * td)
          ad = 0.202 + (0.852 * tb) - (0.007 * td) - (0.357 * tb * td)
  
      # compute hourly radiation
      for alt in altitudes:
          if alt > 0:
              # calculate hourly air mass between top of the atmosphere and earth
              air_mass = get_relative_airmass(alt)
              dir_norm_rad.append(1415 * math.exp(-tb * math.pow(air_mass, ab)))
              dif_horiz_rad.append(1415 * math.exp(-td * math.pow(air_mass, ad)))
          else:
              dir_norm_rad.append(0)
              dif_horiz_rad.append(0)
  
      return dir_norm_rad, dif_horiz_rad